Fuel cell power generation is currently undergoing rapid development both for stationary and transportation applications. In the transportation sector, fuel cells can augment or replace the internal combustion engines in vehicles such as cars, trucks, and buses, while meeting the most stringent emission regulations. In stationary power generation, residential, commercial, and industrial applications are envisioned. In some cases, the hydrogen feedstock will be obtained from hydrogen-rich fuels by on-board or on-site fuel reforming. Generally, the reformate gas includes hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), water (H2O) and a small amount of methane (CH4). However, the CO component needs to be completely removed upstream of a low-temperature fuel cell, such as the PEM fuel cell, because it poisons the anode catalyst, thus degrading the fuel cell performance. CO is also a criterion pollutant.
The low-temperature water-gas shift reaction (LTS), which is represented by the relation CO+H2OCO2+H2, is used to convert carbon monoxide with water vapor to hydrogen and CO2. Currently, a selective CO oxidation reactor is envisioned as the last fuel-processing step upstream of the fuel cell anode. A highly active LTS catalyst would obviate the need for the CO oxidation reactor.
Desired catalyst characteristics include high activity and stability over a wider operating temperature window than is currently possible with the commercial LTS catalysts. Catalysts based on cerium oxide (ceria) are promising for this application. Ceria is presently used as a key component of the three-way catalyst in automotive exhausts. Ceria is also a good choice as a support of both noble metal and base metal oxide catalysts. Ceria participates in redox reactions by supplying and removing oxygen. Metal-ceria systems are several orders of magnitude more active than metal/alumina or other oxide supports for a number of redox reactions. Cu-ceria is more stable than other Cu-based LTS catalysts and at least as active as the precious metal-ceria systems, which are well known for their LTS activity in the catalytic converter.
During the past decade, many studies have established that nanosized gold (Au)-on-reducible oxides have a remarkable catalytic activity for many important oxidation reactions, especially low-temperature CO oxidation, the Water Gas Shift (WGS) reaction, hydrocarbon oxidation, NO reduction and the selective oxidation of propylene to propylene oxide. There is presently no consensus as to the cause of the very high activity of nanoparticles of Au-on-reducible oxides. For example, in oxidation/reduction reactions, some researchers have argued that the oxygen at the interface between the metal and the oxide support is important, while others invoke dissociative O2 adsorption (as oxygen atoms) on very small Au particles but not on bulk Au particles to explain the activity. The unique properties of supported nanoscale Au particles have been correlated to a number of variables, including Au particle size, Au-support interface, the state and structure of the support, as well as the pretreatment of catalysts.
There is a need for an inexpensive and efficient catalyst material having good stability in air and in cyclic operation with respect to the low-temperature water-gas shift reaction.